The present invention pertains to virtual image display systems, where the displayed imagery appears to be located beyond the physical dimensions of the display system. Particularly, the invention pertains to virtual image display systems having compact collimation and relay optics, high resolution, wide field of view, full color mono- or stereoscopic capabilities, and a large exit pupil.
There are a wide variety of techniques for optical displays. These techniques are typically classified into real image and virtual image categories. Most real image methods involve direct viewing of a display surface or viewing of a screen upon which an image is projected. The object or subject being displayed is focused and appears to be located at the physical position of the surface or screen. Difficulties can arise if the display requirements include a large size or high brightness.
In many applications, it is required or desirable that the apparent distance to the subject be larger than is feasible with a real image source. In these cases, virtual image display techniques are used wherein the apparent distance from the observer to the viewed subject is greater than the optical path length from the observer to the display screen. Examples include head up displays, head or helmet mounted displays, and compact simulation displays. These displays include collimation optics to allow a small real image source to appear to be at or near optical infinity. For demanding applications, however, these methods become undesirable due to the size, weight, and complexity of the required optics. Factors include field of view, resolution, distortion, pupil size, and distance between the display and the viewer.
Retroreflective elements have been applied to enhance display performance or to achieve special effects. The use of real image projection screens with retroreflective properties to enhance brightness when viewed within the high gain cone angle is well known. Giordano, in U.S. Pat. No. 3,200,702, issued Aug. 17, 1965, appears to teach that a real image (high spatial resolution) retroreflecting screen can be used to keep two perspective views separate, thereby facilitating autostereoscopic real image projection. This approach is used by Breglia et al. in U.S. Pat. No. 4,348,185, issued Sep. 7, 1982.
Kassies in U.S. Pat. No. 4,509,837, issued Aug. 9, 1985, teaches that a retroreflective array can be used to project a real image of real object, complete with depth and perspective information. This real image is formed between the retroreflector array and the viewer. Although no physical screen is used, the image is considered real since it could be seen if a screen was placed at the focus region. Moss et al. in U.S. Pat. No. 5,035,474, issued Jul. 30, 1991, teach a dual collimator configuration, but not a retroreflective display system.
In the aforementioned examples involving retroreflective screens, the spatial resolution of the retroreflective array is essential to high resolution imaging performance. This places constraints on the achievable angular resolution due to diffractive effects. The presentation of real images for viewing also limits the compactness of such approaches.
In a related art, two basic collimation devices have been used for relaying virtual image display information to a viewer. These devices involve refractive (i.e., lens) optics and catadioptric or reflective (i.e., mirror) optics. Diffractive optics have been considered as well in analogous modes of the refractive and reflective configurations. Flat mirrors and beamsplitters often have been used in conjunction with the collimating device. Such combinations have exhibited disadvantages. For example, in the case of head up displays (HUD's), a large field of view (FOV) in a large head clearance distance necessitates a rather large collimator. Since the collimator must maintain its integrity as an optical element, its large size and a vibration modes, both within the optical element and relative to the rest of the projection system, are troublesome. The vibration modes are minimized at the expense of making the collimator rather thick and heavy thereby leading to further disadvantages and practical limitations.
In all high performance virtual display systems, especially the reflective systems, the optics tend to be complex when trying to compensate for the aberrations introduced by the large collimation elements. Such optics must be large to project the required intermediate image. Some related art discloses a retroreflecting screen that is a real image projection screen where the real image is focused onto the screen surface. One instance is the placement of this screen surface entirely on a "helmet" in a configuration very similar to that which has been regarded as a basic helmet mounted display. However, such display is not collimated and does not share the advantages of the present invention. The primary advantages provided by retroreflectors in this instance are screen gain (e.g., to improve brightness in simulators) and in the helmet display configuration, the separation of the viewability of each eye's image by the other eye.